1. Field of the Invention
This invention generally relates to apparatus for absorbing sound and more specifically to such apparatus that utilizes a piezoelectric anechoic coating.
2. Description of the Prior Art
It often is desirable to influence the acoustic properties of an environment. The following patents, for example, disclose a number of different approaches that have been used for reducing noise levels:
______________________________________ 4,473,906 (1984) Warnaka et al 4,480,333 (1984) Ross 4,677,677 (1987) Eriksson 4,712,247 (1987) Swarte ______________________________________
In each of these patents apparatus samples incoming acoustic energy and produces a compensating signal that can be recombined with the acoustic energy. For example, in the Warnaka et al patent a microphone samples noise passing into a duct. An adaptive filter connected to the microphone generates canceling noise at a location "downstream" in the duct thereby to reduce the overall level of noise emanating from the duct. More specifically the adaptive filter adjusts its output depending on the character of an error signal to produce a canceling sound from a speaker that is nearly equal in amplitude to and 180.degree. out of phase with the source sound. Each of these systems requires a significant and somewhat complex apparatus in the form of microphones, loudspeakers and related electronics equipment. To operate effectively the apparatus must introduce appropriate delays between the receipt of a signal and the transmission of the canceling signal in order to take into account the speed with which the sound travels along a duct.
The use of these systems tends to be limited to applications where the location of the noise source and the direction of sound travel are confined. They do not operate effectively, for example, to reduce echoes emanating from a tank wall particularly where the direction of the acoustic energy is not known and echoes tend to scatter in all directions. A popular solution is the application of a passive, sound absorbing coating to the tank wall. Alberich coatings that consist of air voids in a rubber matrix are examples of such passive coatings. However, the passive coating must have a thickness that constitutes an appreciable fraction of the wavelength of the sound to be absorbed. Consequently, such coatings become impracticably thick at low acoustic frequencies, particularly below 1 kHz where wavelengths in water exceed 1.5 meters.
It has also been proposed to use a sound absorbing coating or layer comprising a layer of piezoelectric transducers. Such coatings are thin and convert a portion of incoming acoustic energy into electrical energy for dissipation in a resistive load. However, piezoelectric transducers are essentially resonant devices. Consequently, the outputs are frequency dependent and effective only over a narrow frequency range.
As an alternative to a single piezoelectric layer, it has been proposed to use two piezoelectric layers or skins in which a first layer acts as a receiver and an active control system drives the second layer to cancel the impinging sound. Although this approach can be effective, it requires the cost of two layers and an active control system with sufficient power to drive the second layer. This can become particularly burdensome and expensive if a large tank wall area is to be modified.
Various problems introduced by the capacitance characteristics of a piezoelectric transducer are recognized in the prior art. The following patents, for example, disclose various circuits for connection to piezoelectric transducers:
______________________________________ 3,390,286 (1968) Gradin et al 3,400,284 (1968) Elazar 4,816,713 (1989) Change ______________________________________
Gradin et al discuss the effects of external capacitance and loads on piezoelectric transducers. They discuss a number of approaches for minimizing the sensitivity of an output signal to transducer capacitance and field effects outside the transducer. More specifically, Gradin et al disclose a temperature compensating capacitor in parallel with a piezoelectric transducer. This capacitor compensates any temperature variations in the piezoelectric transducer capacitance. A low pass filter and an impedance matching network minimize the adverse impact of a load on the transducer.
The Elazar patent discloses a piezoelectric transducer. It includes a temperature dependent capacitor in parallel with the piezoelectric element as a compensating component.
Change discloses a piezoelectric transducer with an electrical feedback circuit for canceling the capacitance of piezoelectric transducers and for increasing voltage gain with a minimal increase in noise levels. A field effect transistor amplifier has a gate connected to one transducer output terminal and a source electrode connected to the other transducer output terminal. Change represents the piezoelectric transducer by a lumped internal crystal capacitance and a stray capacitance. Charges generated by inertia forces on the transducer produce signals that effectively cancel the crystal capacitance by virtue of the use of two capacitors corresponding to the stray capacitance and the inter-electrode capacitance of a field effect transistor amplifier. However, the compensation and gain are dependent.